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Abstract:

A dynamic spinal stabilization bone anchor which supports the spine while
providing for the preservation of spinal motion. The dynamic bone anchor
provides load sharing while preserving range of motion and reducing
stress exerted upon the bone anchors and spinal anatomy. The dynamic bone
anchor includes a deflectable post connected by a ball-joint to a
threaded anchor. Deflection of the deflectable post is controlled by a
centering spring. The force/deflection properties of the dynamic bone
anchor may be adapted to the anatomy and functional requirements of the
patient. The dynamic bone anchor may be used as a component of a dynamic
stabilization system which supports the spine while providing for the
preservation of spinal motion.

Claims:

1. A dynamic spine stabilization device comprising:a bone anchor having a
housing;a cavity in the housing coaxial with the bone anchor;a post
received in the cavity;the post having a retainer at a distal end and a
mount at a proximal end;the retainer being secured in a pocket of the
cavity of the housing such that the post may pivot about the shaped
retainer and rotate about a longitudinal axis of the post; anda spring
positioned in the cavity of the housing between the post and the housing
such that pivoting of the post away from a position in which the
longitudinal axis of the post is coaxial with the bone anchor causes
compression of the spring and such that the spring applies a force upon
the post pushing the post towards a position in which the longitudinal
axis of the post is coaxial with the bone anchor.

2. The device of claim 1, wherein said spring is made of a polymer.

3. The device of claim 1, wherein said spring is made of a superelastic
metal.

4. The device of claim 1, wherein said spring is radially-compressible.

5. The device of claim 1, wherein said spring comprises a plurality of
planar spring elements.

6. The device of claim 1, wherein said housing has a limit surface which
contacts the post upon deflection of said post a first amount from the
position in which the longitudinal axis of the post is coaxial with the
bone anchor.

7. The device of claim 1, wherein:said housing has a limit surface which
contacts the post upon deflection of said post a first amount from the
position in which the longitudinal axis of the post is coaxial with the
bone anchor; andwherein further deflection of said post beyond said first
amount requires a larger load per unit of deflection than deflection of
said post up to said first amount.

8. The device of claim 1, wherein:said housing has a limit surface which
contacts the post upon deflection of said post a first amount from the
position in which the longitudinal axis of the post is coaxial with the
bone anchor; andwherein further deflection of said post beyond said first
amount requires at least double the load per unit of deflection than
deflection of said post up to said first amount.

9. The device of claim 1, including a connector and a vertical rod, with
the connector connecting the vertical rod to the deflection post and with
the deflection under load occurs in the deflection post and to a lesser
degree in the connection rod.

10. The device of claim 1, wherein said post can pivot about a point in
said bone anchor, which point is adapted to be implanted within a
vertebra.

11. The device of claim 1, wherein said deflection post can pivot about a
point in said bone anchor, which point is adapted to be implanted
adjacent a surface of a vertebra.

12. The device of claim 1, wherein said spring has an isotropic deflection
profile.

13. The device of claim 1, wherein said spring has an anisotropic
deflection profile.

14. The device of claim 1 wherein during deflection the deflection post
first is urged against said spring and then is subsequently urged against
a limit surface of said housing.

15. A spine stabilization device comprising:a bone screw having a distal
end adapted to engage a bone;a housing at a proximal end of said bone
anchor;a longitudinal bore in said housing;said bore being aligned with
the bone screw and having an open end and a closed end;a hemispherical
pocket at the closed end of said bore;a deflectable post having a
proximal end, an elongated body and a distal end;the proximal end of said
deflectable post extending from the open end of said longitudinal bore;a
spherical retainer at the distal end of the deflectable post;the
spherical retainer being received in the hemispherical pocket of the
bore;a fastener which secures the spherical retainer in the hemispherical
pocket and allows the deflectable post to pivot and rotate relative to
the bone anchor; anda spring positioned within the bore between the
deflectable post and the housing such that the spring flexibly resists
pivoting of the deflectable post towards the housing.

16. The spine stabilization device of claim 15, further comprising:a limit
surface associated with to the housing and positioned to contact the
deflectable post after a first angle of pivoting of the deflectable post
away from alignment with the bone screw; andwherein the limit surface
resists further pivoting of said deflectable post beyond said first
angle.

17. The spine stabilization device of claim 15, further comprising:a limit
surface associated with the housing and positioned to contact the
deflectable post after a first angle of pivoting of the deflectable post
away from alignment with the bone screw; andwherein deflection of the
proximal end of the deflectable post after contact between the
deflectable post and the limit surface requires at least double the load
per unit of deflection than deflection of said post prior to contact
between the deflectable post and the limit surface.

18. The spine stabilization device of claim 15, wherein said spring is
made of PEEK.

19. The spine stabilization device of claim 15, wherein the spring
comprises a first portion in contact with the deflectable post, a second
portion in contact with the housing and a flexible portion between the
first portion and the second portion which is elastically deformed by
pivoting of the deflectable post towards the housing.

20. The spine stabilization device of claim 15, wherein the spring
comprises a first portion in contact with the deflectable post, a second
portion in contact with the housing and a flexible portion between the
first portion and the second portion which is elastically deformed by
pivoting of the deflectable post towards the housing and wherein a third
portion comprises a plurality of lever arms.

21. The spine stabilization device of claim 15, wherein the spring
comprises a first portion in contact with the deflectable post, a second
portion in contact with the housing and a flexible portion between the
first portion and the second portion which is elastically deformed by
pivoting of the deflectable post towards the housing and wherein the
third portion comprises a coil.

22. The spine stabilization device of claim 15, wherein the spring
comprises one or more spring washers.

23. The spine stabilization device of claim 15, wherein the spring is
radially compressed by deflection of the deflectable post.

24. An implantable spine stabilization device comprising:an elongated bone
anchor having a distal end and a proximal end;a post having a distal end
and a proximal end;a ball-joint which secures the distal end of the post
to the proximal end of the bone anchor such that the post can pivot
relative to the bone anchor;a tubular extension of the bone anchor which
extends over a distal portion of the post; anda spring disposed between
the distal portion of the post and the tubular extension of the bone
anchor whereby the spring biases the post into alignment with the bone
anchor.

25. The spine stabilization device of claim 24, further comprising:a limit
surface associated with the tubular extension housing and positioned to
contact the post when the post pivots through a first angle from
alignment with the bone anchor; andwherein the limit surface resists
pivoting of said post beyond said first angle.

26. The spine stabilization device of claim 24, wherein said spring has a
ring having an outer diameter sized to fit with the tubular extension and
a central aperture sized to receive the post wherein the central aperture
is substantially surrounded by a plurality of lever arms connected to the
ring.

27. The spine stabilization device of claim 24, wherein said spring is
made of PEEK.

28. The spine stabilization device of claim 24, wherein said spring
comprises one or more planar spring elements.

[0033]All of the afore-mentioned patent applications are incorporated
herein by reference in their entireties.

BACKGROUND OF INVENTION

[0034]Back pain is a significant clinical problem and the costs to treat
it, both surgical and medical, are estimated to be over $2 billion per
year. One method for treating a broad range of degenerative spinal
disorders is spinal fusion. Implantable medical devices designed to fuse
vertebrae of the spine to treat have developed rapidly over the last
decade. However, spinal fusion has several disadvantages including
reduced range of motion and accelerated degenerative changes adjacent the
fused vertebrae.

[0035]Alternative devices and treatments have been developed for treating
degenerative spinal disorders while preserving motion. These devices and
treatments offer the possibility of treating degenerative spinal
disorders without the disadvantages of spinal fusion. However, current
devices and treatments suffer from disadvantages e.g., complicated
implantation procedures; lack of flexibility to conform to diverse
patient anatomy; the need to remove tissue and bone for implantation;
increased stress on spinal anatomy; insecure anchor systems; poor
durability, and poor revision options. Consequently, there is a need for
new and improved devices and methods for treating degenerative spinal
disorders while preserving motion.

SUMMARY OF INVENTION

[0036]The present invention includes a spinal implant system and methods
that can dynamically stabilize the spine while providing for the
preservation of spinal motion. Embodiments of the invention provide a
dynamic stabilization system which includes: versatile components,
adaptable stabilization assemblies, and methods of implantation. An
aspect of embodiments of the invention is the ability to stabilize two,
three and/or more levels of the spine by the selection of appropriate
components of embodiments of the invention for implantation in a patient.
Another aspect of embodiments of the invention is the ability to
accommodate particular anatomy of the patient by providing a system of
versatile components which may be customized to the anatomy and needs of
a particular patient and procedure. Another aspect of the invention is to
facilitate the process of implantation and minimize disruption of tissues
during implantation.

[0037]Thus, the present invention provides new and improved systems,
devices and methods for treating degenerative spinal disorders by
providing and implanting a dynamic spinal stabilization assembly which
supports the spine while preserving motion. These and other objects,
features and advantages of the invention will be apparent from the
drawings and detailed description which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038]FIGS. 1A and 1B are perspective views of a deflection system
component mounted to an anchor system component according to an
embodiment of the present invention.

[0039]FIG. 1c is a perspective view of a connection system component
mounted to an anchor system component according to an embodiment of the
present invention.

[0040]FIG. 1D is a perspective view of a different connection system
component mounted to an anchor system component according to an
embodiment of the present invention.

[0041]FIG. 1E is a posterior view of an anchor system for a multi-level
dynamic stabilization assembly utilizing the anchor components of FIGS.
1A to 1D according to an embodiment of the present invention.

[0042]FIG. 1F is a posterior view of a multi-level dynamic stabilization
assembly utilizing the components of FIGS. 1A to 1E according to an
embodiment of the present invention.

[0043]FIG. 2A is an exploded view of a deflection rod according to an
embodiment of the present invention.

[0045]FIG. 2C is a perspective view of the deflection rod of FIG. 2A, as
assembled.

[0046]FIG. 2D is a sectional view of the deflection rod of FIGS. 2A and
2C.

[0047]FIG. 2E is a partial sectional view of the deflection rod of FIGS.
2A and 2C.

[0048]FIGS. 2F and 2G are sectional views of the deflection rod of FIGS.
2A and 2C showing deflection of the post.

[0049]FIG. 3A is an exploded view of an alternative deflection rod
according to an embodiment of the present invention.

[0050]FIG. 3B is a perspective view of the deflection rod of FIG. 3A, as
assembled.

[0051]FIG. 3c is a perspective view illustrating engagement of a bone
anchor by a driver.

[0052]FIG. 3D is a sectional of the assembled deflection rod of FIG. 3B.

[0053]FIG. 4A is an exploded view of an alternative deflection rod
according to an embodiment of the present invention.

[0054]FIG. 4B is an enlarged view of the spring of the deflection rod of
FIG. 4A.

[0055]FIG. 4c is a perspective view of the deflection rod of FIG. 4A, as
assembled.

[0056]FIG. 4D is a sectional view of the deflection rod of FIG. 4A, as
assembled.

[0057]FIG. 5A is an exploded view of an alternative deflection rod
according to an embodiment of the present invention.

[0058]FIG. 5B is a sectional view of the deflection rod of FIG. 5A, as
assembled.

[0059]FIG. 5c is an enlarged view of a spring element of the deflection
rod of FIG. 5A.

[0060]FIGS. 5D-5G show views of alternative spring elements suitable for
use in the deflection rod of FIGS. 5A and 5B.

[0061]FIG. 6A is a transverse sectional view of a vertebra illustrating
the implantation of a deflection rod and bone anchor according to an
embodiment of the invention.

[0062]FIG. 6B is a lateral view of the spine illustrating a single level
dynamic stabilization system utilizing a deflection rod according to an
embodiment of the invention.

[0063]FIGS. 7A and 7B are perspective views of an alternate mount for
connecting a deflection rod to a vertical rod according to an embodiment
of the present invention.

[0064]FIGS. 8A-8E are perspective views of alternative combinations of
deflection rods and bone anchors according to embodiments of the present
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0065]The present invention includes a versatile spinal implant system and
methods which can dynamically stabilize the spine while providing for the
preservation of spinal motion. Alternative embodiments can be used for
spinal fusion. An aspect of the invention is restoring and/or preserving
the natural motion of the spine including the quality of motion as well
as the range of motion. Still, another aspect of the invention is
providing for load sharing and stabilization of the spine while
preserving motion.

[0066]Another aspect of the invention is to provide a modular system which
can be customized to the needs of the patient. Another aspect of
embodiments of the invention is the ability to stabilize two, three
and/or more levels of the spine by the selection of appropriate
components for implantation in a patient. Another aspect of the invention
is the ability to provide for higher stiffness and fusion at one level or
to one portion of the spine while allowing for lower stiffness and
dynamic stabilization at another adjacent level or to another portion of
the spine. Embodiments of the invention allow for fused levels to be
placed next to dynamically-stabilized levels. Such embodiments of the
invention enable vertebral levels adjacent to fusion levels to be
shielded by providing a transition from a rigid fusion level to a
dynamically stable, motion preserved, and more mobile level.

[0067]Embodiments of the present invention provide for assembly of a
dynamic stabilization system which supports the spine while providing for
the preservation of spinal motion. The dynamic stabilization system has
an anchor system, a deflection system, a vertical rod system and a
connection system. The anchor system anchors the construct to the spinal
anatomy. The deflection system provides dynamic stabilization while
reducing the stress exerted upon the bone anchors and spinal anatomy. The
vertical rod system connects different levels of the construct in a
multilevel assembly and may in some embodiments include compound
deflection rods. The connection system includes coaxial connectors and
offset connectors which adjustably connect the deflection system,
vertical rod system and anchor system allowing for appropriate, efficient
and convenient placement of the anchor system relative to the spine.
Alternative embodiments can be used for spinal fusion.

[0068]Embodiments of the invention include a construct with an anchor
system, a deflection system, a vertical rod system and a connection
system. The deflection system provides dynamic stabilization while
reducing the stress exerted upon the bone anchors and spinal anatomy. The
anchor system anchors the deflection system to the spine. The connection
system connects the deflection system to the vertical rod system. The
vertical rod system connects dynamic stabilization system components on
different vertebra to provide load sharing and dynamic stabilization.

[0069]Embodiments of the present invention include a deflection rod which
provides load sharing while preserving range of motion and reducing
stress exerted upon the bone anchors and spinal anatomy. The deflection
rod includes a deflectable post mounted within a bone anchor. Deflection
of the deflectable post is controlled by a spring. A contact surface of
the deflection rod is positioned to limit deflection of the deflectable
post. The force-deflection properties of the deflection rod may be
adapted and/or customized to the anatomy and functional requirements of
the patient by changing the properties of the spring. Different
deflection rods having different force-deflection properties may be
utilized in different patients or at different spinal levels within the
same patient depending upon the anatomy and functional requirements.
Moreover deflection rods may be utilized at a spinal level with fusion at
an adjacent spinal level.

[0070]Common reference numerals are used to indicate like elements
throughout the drawings and detailed description; therefore, reference
numerals used in a drawing may or may not be referenced in the detailed
description specific to such drawing if the associated element is
described elsewhere. The first digit in a reference numeral indicates the
series of figures in which the referenced item first appears.

[0071]The terms "vertical" and "horizontal" are used throughout the
detailed description to describe general orientation of structures
relative to the spine of a human patient that is standing. This
application also uses the terms proximal and distal in the conventional
manner when describing the components of the spinal implant system. Thus,
proximal refers to the end or side of a device or component closest to
the hand operating the device, whereas distal refers to the end or side
of a device furthest from the hand operating the device. For example, the
tip of a bone screw that enters a bone would conventionally be called the
distal end (it is furthest from the surgeon) while the head of the screw
would be termed the proximal end (it is closest to the surgeon).

Dynamic Stabilization System

[0072]FIGS. 1A-1F introduce components of a dynamic stabilization system
according to an embodiment of the present invention. The components
include anchor system components, deflection rods, vertical rods and
connection system components, including for example coaxial and offset
connectors. The components may be implanted and assembled to form a
dynamic stabilization system appropriate for the anatomical and
functional needs of a patient.

[0073]FIG. 1A shows a bone anchor 102 and a deflection rod 104 connected
to a vertical rod 106 by a ball joint 108. In other embodiments, the
vertical rod may be mounted directly to the deflection rod. Deflection
rod 104 is an example of a component of the deflection system. Deflection
rod 104 is a component having controlled flexibility which allows for
load sharing. The deflection rod 104 provides stiffness and support where
needed to support the loads exerted on the spine during normal spine
motion, which loads, the soft tissues of the spine are no longer able to
accommodate since these spine tissues are either degenerated or damaged.
Load sharing is enhanced by the ability to select the appropriate
stiffness of the deflection rod in order to match the load sharing
characteristics desired. For embodiments of this invention, the terms
"deflection rod" and "loading rod" can be used interchangeably.
Deflection rods, deflection rod mountings and alternative deflection rods
are described in more detail below.

[0074]Deflection rod 104 includes a deflectable post 105 which may deflect
relative to a collar 107. Collar 107 is adapted to secure the deflectable
post 105 to bone anchor 102. Collar 107 secures deflection rod 104 within
cavity 132 of bone anchor 102. In other embodiments, the deflection rod
may be integrated with a bone anchor. When received in cavity 132, collar
107 is secured into a fixed position relative to bone anchor 102.
Deflectable post 105 may still deflect in a controlled manner relative to
bone anchor 102 thereby provide for load sharing while preserving range
of motion of the patient. The stiffness/flexibility of deflection of the
deflectable post 105 relative to the bone anchor 102 may be controlled
and/or customized as will be described below.

[0075]As shown in FIG. 1A, collar 107 is designed to secure deflection rod
104 within a cavity 132 of bone anchor 102. As shown in FIG. 1A, a
threaded aperture 142 extends obliquely through collar 107. The threaded
aperture 142 receives a locking set screw 144 which, when seated (FIG.
1B), engages the housing 130 of bone anchor 102. Locking set screw 144 is
positioned within threaded aperture 142 through collar 107. The locking
set screw 144 thereby secures the deflection rod 104 in place within the
housing 130 of bone anchor 102.

[0076]Bone anchor 102 is an example of a component of the anchor system.
Bone anchor 102 includes a bone screw 120 and housing 130. As shown in
FIG. 1A, bone anchor 102 is a bone screw 120 having one or more threads
124 which engage a bone to secure the bone anchor 102 onto a bone. The
anchor system may include one or more alternative bone anchors known in
the art e.g. bone hooks, expanding devices, barbed devices, threaded
devices, adhesive and other devices capable of securing a component to
bone instead of or in addition to bone screw 120.

[0077]As shown in FIG. 1A, deflection rod 104 is oriented in a co-axial,
collinear or parallel orientation to bone anchor 102. This arrangement
simplifies implantation, reduces trauma to structures surrounding an
implantation site, and reduces system complexity. Arranging the
deflection rod 104 co-axial with the bone anchor 102 can substantially
transfer a moment (of) force applied by the deflectable post 105 from a
moment force tending to pivot or rotate the bone anchor 102 about the
axis of the shaft, to a moment force tending to act perpendicular to the
axis of the shaft. The deflection rod can, thereby, effectively resist
repositioning of the deflection rod and/or bone anchor 102 without the
use of locking screws or horizontal bars to resist rotation. Further
examples of coaxial deflection rods are provided below. Each of the
deflection rods described herein may be used as a component of a dynamic
stabilization system.

[0078]As shown in FIG. 1A, bone anchor 102 includes a housing 130 at the
proximal end. Housing 130 includes a cavity 132 for receiving deflection
rod 104. Cavity 132 is coaxial with threaded bone screw 120. Housing 130
also comprises a groove 134 for securing deflection rod 104 within
housing 130. As shown in FIG. 1A, groove 134 is located at the proximal
end of housing 130. Groove 134 is designed to be engaged by the locking
mechanism of a component mounted within cavity 132. For example, groove
134 is designed to be engaged by locking set screw 144 of deflection rod
104. When deflection rod 104 has been positioned within cavity 132 of
bone anchor 102 as shown in FIG. 1B, locking set screw 144 is tightened
to engage groove 134 of housing 130, thus, securing deflection rod 104
within housing 130. Alternative mechanisms and techniques may be used to
secure the deflection rod to the bone anchor including for example,
welding, soldering, bonding, and/or mechanical fittings including
threads, snap-rings, locking washers, cotter pins, bayonet fittings or
other mechanical joints.

[0079]Bone anchor 102 also includes a coupling 136 to which other
components may be mounted. As shown in FIG. 1A, coupling 136 is the
external cylindrical surface of housing 130. Housing 130 thus provides
two mounting positions, one coaxial mounting position and one external
(or offset) mounting position. Thus, a single bone anchor 102 can serve
as the mounting point for one, two or more components. A deflection rod
104 may be coaxially mounted in the cavity 132 of the housing and one or
more additional components may be externally mounted to the outer surface
of the housing-coupling 136. For example, a component of the connection
system may be mounted to the outer surface 136 of the housing--such a
connector may be called an offset head or offset connector. In some
applications, a component of the connection system may be
coaxially-mounted in the cavity 132 in place of a deflection rod
104--such a connector may be called a coaxial head or coaxial connector.

[0080]It is desirable to have a range of different connectors which are
compatible with the anchor system and deflection system. The connectors
may have different attributes, including for example, different degrees
of freedom, range of motion, and amount of offset, which attributes may
be more or less appropriate for a particular relative orientation and
position of two bone anchors and/or patient anatomy. It is desirable that
each connector be sufficiently versatile to connect a vertical rod to a
bone anchor in a range of positions and orientations while being simple
for the surgeon to adjust and secure. It is desirable to provide a set of
connectors which allows the dynamic stabilization system to be assembled
in a manner that adapts a particular dynamic stabilization assembly to
the patient anatomy rather than adapting the patient anatomy for
implantation of the assembly (for example by removing tissue\bone to
accommodate the system). In a preferred embodiment, the set of connectors
comprising the connection system have sufficient flexibility to allow the
dynamic stabilization system to realize a suitable dynamic stabilization
assembly in all situations that will be encountered within the defined
patient population.

[0081]In some embodiments of the present invention, a connection system
component, e.g. a polyaxial connector may be mounted in the cavity 132 of
a bone anchor 102 to secure the bone anchor to a vertical rod. For
example, FIG. 1c shows coaxial head 150 which is a polyaxial connector
which is coaxially mounted within the cavity 132 of the housing 130 of
bone anchor 102. Coaxial head 150 is an example of a coaxial head or
coaxial connector. Bone anchor 102 is the same bone anchor previously
described with respect to FIGS. 1A and 1B. Coaxial head 150 comprises a
rod 152 which is designed to fit within cavity 132 of housing 130.
Coaxial head 150 also comprises a collar 154 and locking set screw 156.
Locking set screw 156 is configured to engage groove 134 of bone anchor
102 in the same way as locking set screw 144 of deflection rod 104. Rod
152 and cavity 132 may, in some case, be circular in section (e.g.
cylindrical), in which case rod 152 can rotate within cavity 132 until
locked into place by fastener 134. In alternative embodiments, rod 152
may be polygonal in section such that it fits in one of a fixed number of
possible positions.

[0082]Referring again to FIG. 1c, attached to rod 152 of coaxial head 150
is a yoke 164. Yoke 164 is connected to a ball 165 by a hexagonal pin
162. A saddle 163 is also mounted to ball 165 such that saddle 163 can
pivot about two orthogonal axes relative to yoke 164. Saddle 163 has an
aperture 168 through which a vertical rod may be passed. On one side of
aperture 168 is a plunger 169. On the other side of aperture 168 is a
locking set screw 167. When a vertical rod 106 (not shown) is positioned
within aperture 168 and locking set screw 167 is tightened down, the
locking set screw 167 forces the vertical rod 106 down onto the plunger
169. Plunger 169 is, in turn, forced down by the vertical rod 106 against
ball 165. Plunger 169 engages ball 165, and ball 165 engages hexagonal
pin 162, to lock saddle 163 in position relative to yoke 164 and secure a
rod (e.g. vertical rod 106) to saddle 163. In this way, tightening set
screw 167 secures the vertical rod 106 to the coaxial head 150 and also
locks orientation of the coaxial head 150.

[0083]The ability to coaxially mount coaxial head 150 to a bone anchor 102
has several advantages over a standard polyaxial bone screw in which a
polyaxial connector is an integral part of the device and may not be
removed or exchanged. The bone anchor 102 is simpler to install and there
is no risk of damage to the polyaxial connector during installation. A
single coaxial head 150 can be manufactured and designed to mount to a
range of different bone anchors thus allowing bone anchors to be selected
as appropriate for the patient anatomy. After the bone anchor is
installed the orientation of the yoke 164 can be adjusted without
changing the screw depth (this is not possible in a standard polyaxial
bone screw without also turning the screw). After the bone anchor is
implanted, one of a range of different coaxial heads may be installed
without requiring removal of the bone anchor. Likewise, if a revision is
required the coaxial head may be exchanged for a different component
without necessitating removal of the bone anchor 102.

[0084]As described above, bone anchor 102 has a housing which can accept
one coaxially-mounted component (e.g. a coaxial head) and one
externally-mounted component (e.g. an offset connector). FIG. 1D shows a
component of the connection system which may be mounted externally to the
housing 130 of bone anchor 102 in conjunction with a coaxially-mounted
component. FIG. 1D shows a perspective view of offset connector 170
mounted externally to housing 130 of bone anchor 102 in which a
deflection rod 104 is coaxially mounted. Connector 170 may be termed an
offset head or offset connector.

[0085]Offset connector 170 comprises six components and allows for two
degrees of freedom of orientation and two degrees of freedom of position
in connecting a vertical rod to a bone anchor. The six components of
offset connector 170 are dowel pin 172, pivot pin 174, locking set screw
176, plunger 178, clamp ring 180 and saddle 182. Saddle 182 has a slot
184 sized to receive a rod which may be a vertical rod, e.g. vertical rod
106 of FIG. 1A. Locking set screw 176 is mounted at one end of slot 184
such that it may be tightened to secure a rod within slot 184.

[0086]Clamp ring 180 is sized such that, when relaxed it can slide freely
up and down the housing 130 of bone anchor 102 and rotate around the
housing 130. However, when locking set screw 176 is tightened on a rod,
the clamp ring 180 grips the housing and prevents the offset connector
170 from moving in any direction. Saddle 182 is pivotably connected to
clamp ring 180 by pivot pin 174. Saddle 182 can pivot about pivot pin
174. However, when locking set screw 176 is tightened on a rod, the
plunger 178 grips the clamp ring 180 and prevents further movement of the
saddle 182. In this way, operation of the single set screw 176 serves to
lock the clamp ring 180 to the housing 130 of the bone anchor 102, fix
saddle 182 in a fixed position relative to clamp ring 180 and secure a
rod within the slot 184 of offset connector 170.

[0087]The above-described coaxial connector and offset connector are
provided by way of example only. Alternative embodiments of coaxial heads
and offset connectors can be found in U.S. Provisional Patent Application
No. 61/100,625, filed Sep. 26, 2008 entitled "Versatile Assembly
Components And Methods For A Dynamic Spinal Stabilization System"
(Attorney Docket No.: SPART-01043US0) which is incorporated by reference.
These coaxial heads and offset connectors may be used in conjunction with
the components herein described to permit assembly of a dynamic
stabilization system appropriate to the functional needs and anatomy of a
particular patient. In addition screws having an integrated connector may
also be utilized to anchor components of the dynamic stabilization system
in fixed relationship to a vertebra, for example polyaxial screws.

[0088]The components of the dynamic stabilization system may be assembled
and implanted in the spine of a patient to provide a multilevel dynamic
stabilization assembly which provides dynamic stabilization of the spine
and load sharing. In some embodiments, the first step is implantation of
bone anchors in the vertebrae. In other embodiments, the bone anchors may
be implanted with the deflection rod/connection component already
installed and/or built in.

[0089]FIG. 1E, shows three adjacent vertebrae 191, 192 and 193. As a
preliminary step, bone anchors 102a, 102b and 102c have been implanted in
the vertebrae 191, 192 and 193 on the right side of the spinous process
194 between the spinous process 194 and the transverse process 195. A
driver is inserted into the cavity 132a, 132b, 132c in order to drive the
threaded portion of each bone anchor into the bone. In preferred
procedures, the bone anchor is directed so that the threaded portion is
implanted within one of the pedicles 196 angled towards the vertebral
body 197. The threaded region of each bone anchor is fully implanted in
the vertebrae 191, 192 and 193. A driver may alternatively and/or
additionally engage the exterior surface of housing 130 in order to
implant the bone anchor.

[0090]As shown in FIG. 1E, the housings 130a, 130b, 130c of each bone
anchor remain partly or completely exposed above the surface of the
vertebrae so that one or more of a connection system component and
deflection component can be secured to each bone anchor 102a, 102b and
102c. Coaxial components may be coaxially-mounted inside each of cavities
132a, 132b, and 132c. Offset heads/connectors may also be
externally-mounted to the outside surface of each of housings 130a, 130b
and 130c. Note that bone anchors are also implanted on the left side of
the spine.

[0091]After installation of the bone anchors, the deflection system
components, vertical rod systems components and connection system
components may be installed and assembled. FIG. 1F shows one way to
assemble deflection system components and connection system components.
As shown in FIG. 1F, a coaxial head 150 is installed in bone anchor 102c.
An offset connector 170 is mounted externally to the housing of bone
anchor 102b. A deflection rod 104a is coaxially mounted in the housing of
bone anchor 102a. A deflection rod 104b is coaxially mounted in the
housing of bone anchor 102b. A vertical rod 106a is connected at one end
to deflection rod 104a by ball joint 108a. Vertical rod 106a is connected
at the other end by in-line connector 170 to bone anchor 102b. A second
vertical rod 106b is connected at one end to deflection rod 104b by ball
joint 108b. Vertical rod 106b is connected at the other end by coaxial
head 160 to bone anchor 102c.

[0092]The dynamic stabilization assembly 190 of FIG. 1F thus has a
vertical rod 106a, 106b stabilizing each spinal level (191-192 and
192-193). Each of the vertical rods 106a, 106b is secured rigidly at one
end to a bone anchor (102b, 102c). Each of the vertical rods 106a, 106b
is secured at the other end by a ball joint to a deflection rod 108a,
108b thereby allowing for some movement and load sharing by the dynamic
stabilization assembly. Offset connector 170 and coaxial head 150 permit
assembly of dynamic stabilization assembly 190 for a wide range of
different patient anatomies and/or placements of bone anchors 102a, 102b
and 102c. An identical or similar dynamic stabilization assembly would
preferably be implanted on the left side of the spine. It should be noted
that dynamic stabilization assembly 190 does not require horizontal bars
or locking screws thereby reducing the exposure of tissue and/or bone to
foreign bodies compared to systems with this additional hardware. The
dynamic stabilization assembly of FIG. 1F, thereby, has a small
footprint, potentially reducing the amount of displacement of tissue
and/or bone, reducing trauma to tissue and/or bone during surgery.
Further, the smaller footprint can reduce the amount of tissue that needs
to be exposed during implantation.

[0093]The particular dynamic stabilization assembly shown in FIG. 1F is
provided by way of example only. It is an aspect of preferred embodiments
of the present invention that a range of components be provided and that
the components may be assembled in different combinations and
organizations to create different assemblies suitable for the functional
needs and anatomy of different patients. Also, deflection rods having
different force deflection characteristics may be incorporated at
different spinal levels in accordance with the anatomical and functional
requirements. Dynamic stabilization may be provided at one or more motion
segments and in some cases dynamic stabilization may be provided at one
or more motion segments in conjunction with fusion at an adjacent motion
segment. Particular dynamic stabilization assemblies may incorporate
combinations of the bone anchors, vertical rods, deflection rods, offset
and coaxial connectors described herein, in the related applications
incorporated by reference, and standard spinal stabilization and/or
fusion components, for example screws, rods and polyaxial screws.

Deflection Rods/Loading Rods

[0094]One feature of embodiments of the present invention is the load
sharing and range of motion provided by the deflection system and
deflection rods of the deflection system. The deflection rod provides
stiffness and support where needed to support the loads exerted on the
spine during normal spine motion thereby recovering improved spine
function without sacrificing all motion. The deflection rod also isolates
the anchor system components from forces exerted by the dynamic
stabilization assembly; thereby reducing stress on the bone anchors and
the bone to which they are attached. Moreover, by selecting the
appropriate stiffness of the deflection rod to match the physiology of
the patient and the loads that the patient places on the spine, a better
outcome is realized for the patient.

[0095]The deflection rods of the present invention include in particular
embodiments a deflectable post, a spring and a mounting/securing device.
The deflectable post and mounting/securing device are typically made of
biocompatible metal or metals, e.g. titanium and stainless steel. The
spring is made of an elastic material, which may be a polymer or a metal.
Suitable polymers include, for example, PEEK and Bionate®. Suitable
metals include, for example, titanium, steel and Nitinol. The
mounting/securing device secures the deflection rod to an anchoring
device, for example, a bone screw, in a manner which allows deflection of
the deflectable post. In some embodiments, the deflection rod is
integrated with an anchoring device rather than selectably and/or
removably mounted.

[0096]The deflectable post is configured to connect to the vertical rod
system. The deflectable post may deflect relative to the anchoring device
by compressing the spring. The deformation of the spring imparts
force-deflection characteristics to the deflectable post. The movement of
the deflectable post relative to the anchoring device allows controlled
movement of the bone anchor (and vertebra in which it is implanted)
relative to the vertical rod system. The deflection rod, thus, supports
the vertebrae to which the bone anchors are attached while allowing
movement of the vertebrae thereby providing for dynamic stabilization of
the spine. In a dynamic stabilization assembly incorporating the
deflection rod, the load sharing and deflection is provided by the
deflection rod and to a lesser degree or not in the vertical rod such as
the vertical rod 106 of FIG. 1A.

[0097]Deflection rods can be manufactured in a range from stiff
configurations to compliant configurations by appropriate selection of
the design, materials and dimensions of the post, spring and
shield/housing. In particular, the spring rate of the spring can be
adjusted to control the stiffness/flexibility of the deflection rod.
Deflection rods having a particular stiffness/flexibility may be selected
for use in a dynamic stabilization assembly based upon the physiological
needs of a particular patient. In a preferred embodiment, deflection rod
stiffness/flexibility is selected to provide load sharing in conjunction
with from 50% to 100% of the normal range of motion of a patient and more
preferably 70% to 100% of the normal range of motion of a patient.

[0098]In some cases, certain of the deflection rods of a dynamic
stabilization assembly can have a different stiffness or compliance than
other of the deflection rods. Thus, in the same assembly, a first
deflection rod can have a first flexibility or stiffness or rigidity, and
a second deflection rod can have a second different flexibility or
stiffness or rigidity depending on the needs of the patient. Particular
embodiments of a dynamic stabilization assembly may utilize deflection
rods having different deflection properties for each level and/or side of
the dynamic stabilization assembly. In other words, one portion of a
dynamic stabilization assembly may offer more resistance to movement than
the other portion based on the design and selection of different on the
deflection rods having different stiffness characteristics, if that
configuration benefits the patient.

[0099]FIGS. 2A through 2G illustrate the design and operation of a first
embodiment of a deflection rod according to an embodiment of the present
invention. FIG. 2A shows an exploded view of deflection rod 200.
Deflection rod 200 includes retainer 202, deflectable post 204, spring
206, shield 208, collar 210, screw 212 and ball 214. Deflection rod 200
connects to vertical rod 216 at a ball joint which includes ball 214,
pocket 218 and cap 220. Shield 208 and collar 210 are securely attached
to each other (or formed in one piece). A threaded aperture 211 passes
obliquely through collar 210. Threaded aperture 211 is configured to
receive a screw 212. Spring 206 is made of a compliant material which
permits movement of deflectable post 204 relative to shield 208.

[0100]Retainer 202 may be a ball-shaped retainer 202 as shown. Retainer
202 may be formed in one piece with deflectable post 204 or may be
securely attached to deflectable post 204. The retainer 202 may be
attached by laser welding, soldering or other bonding technology. For
example, retainer 202 in the form of a ball, disk, plate or other shape
may be laser welded to the distal end of deflectable post 204.
Alternatively, retainer 202 may mechanically engage the deflectable post
204 using, for example, threads. For example, a lock ring, toothed
locking washer, cotter pin or other mechanical device can be used to
secure deflectable post 204 within shield 208.

[0101]FIG. 2B shows an enlarged view of spring 206. As shown in FIG. 2B,
spring 206 comprises a ring-shaped base 260 from which extends a
plurality of lever arms 262. The lever arms extend upwards from base 260
and extend in towards the central axis of ring-shaped base 260. The lever
arms 262 define an aperture 264 which is large enough for the passage of
deflectable post 204 (not shown). The material of spring 206 is selected
such that the lever arms resist bending away from the position shown.
Ring-shaped base 260 also includes rim 205 which is engaged by the lower
edge of the shield 208 (See FIG. 2A). In some embodiments, spring 206 may
be formed separately from deflection rod 200. For example, deflectable
post 204 and spring 206 may be press fit into shield 208. Alternatively
or additionally, a biocompatible adhesive may be used to bond the spring
206 to the shield 208.

[0102]The stiffness of deflection rod 200 is affected by the spring rate
of spring 206. The stiffness of the deflection rod 200 can be changed for
example by increasing the spring rate of spring 206 and conversely, the
stiffness may be reduced by decreasing the spring rate of spring 206. The
spring rate of the spring 206 can be, for example, increased by
increasing the thickness of the lever arms 262 and/or decreasing the
length of the lever arms 262. Alternatively and/or additionally changing
the materials of the spring 206 can also affect the spring rate. For
example, making spring 206 out of stiffer material increases the spring
rate and thus reduces deflection of deflectable post 204 for the same
amount of load--all other factors being equal. Spring 206 is preferably
made of a biocompatible polymer or metal. Spring 206 may, for example, be
made from PEEK, Bionate®, Nitinol, steel and/or titanium.

[0103]Spring 206 may have the same spring rate in each direction of
deflection of the deflectable post (isotropic). The spring 206 may have
different spring rates in different directions of deflection of the
deflectable post (anisotropic). For example, the spring 206 can be
designed to have a different spring rate in different directions by
adjusting, for example, the length, thickness and/or material of the
lever arms 262 in one direction compared to another direction. A
deflection rod 200 incorporating an anisotropic spring would have
different force-deflection characteristics imparted to it by the spring
206 in different directions.

[0104]The stiffness of the deflection rod 200 is also affected by factors
beyond the spring rate of spring 206. By changing the dimensions and or
geometry of the deflectable post 204, spring 206 and the shield 208, the
deflection characteristics of the deflection rod 200 can be changed. For
example, the stiffness of the deflection rod 200 can be increased by
increasing the distance from the pivot point of the deflectable post 204
to the point of contact between the lever arms 262 surrounding aperture
264 and the deflectable post 204. Conversely, the stiffness of the
deflection rod 200 can be decreased by decreasing the distance from the
pivot point of the deflectable post 204 to the point of contact between
the lever arms 262 surrounding aperture 264 and the deflectable post 204.

[0105]The stiffness of the deflection rod may thus be varied or customized
according to the needs of a patient by controlling the material and
design of spring 206 and defection rod 200. The deflection
characteristics of the deflection rod 200 can be configured to approach
the natural dynamic motion of the spine, while giving dynamic support to
the spine in that region. It is contemplated, for example, that the
deflection rod can replicate a 70% range of motion and flexibility of the
natural intact spine, a 50% range of motion and flexibility of the
natural intact spine and a 30% range of motion and flexibility of the
natural intact spine.

[0106]One feature of the present invention is to allow the efficient
manufacture of a range of deflection rods having a range of different
force-deflection characteristics. This can readily be accomplished by
manufacturing a range of springs having different force-deflection
characteristics and leaving the remainder of the components unchanged. In
this way, a range of deflection rods may be manufactured with a small
number of unique parts. In some cases, a kit is provided to a doctor
having a set of deflection rods with different force-deflection
characteristics from which the doctor may select the deflection rods most
suitable for a particular patient. In other cases, the surgeon may select
deflection rods prior to the procedure based upon pre-operative
assessment.

[0107]Referring now to FIG. 2C, which shows a perspective view of a fully
assembled deflection rod 200. When assembled, deflectable post 204 is
positioned within spring 206 which is positioned within shield 208. A rim
205 on the outside surface of spring 206 is engaged by the lower edge of
the shield 208. Ball-shaped retainer 202 is received in a partially
spherical pocket 207 (See FIG. 2D) in the lower edge of spring 206.
Deflectable post 204 may thus pivot in any direction about the center of
ball-shaped retainer 202 as shown by arrows 230. (The lower half of
ball-shaped retainer 202 is adapted to be received in a hemispherical
pocket of the bone anchor (See FIG. 2D). The deflectable post 204 can
also rotate about the longitudinal axis of the post and the bone anchor
as shown by arrow 232.

[0108]Referring again to FIG. 2C, ball 214 is connected to the proximal
end of deflectable post 204 to provide a component of a ball joint for
connecting deflection rod 200 to a vertical rod 216. Ball 214 may be
formed in one piece with deflectable post 204 or may be securely attached
to deflectable post 204 using a joint, for example, a threaded joint,
welded joint or adhesive joint. Retainer 202 is attached to the distal
end of deflectable post 204 to prevent deflectable post 204 from being
pulled out of spring 206. A cap 220 secures ball 214 within the pocket of
vertical rod 216 creating a ball joint 222 which allows vertical rod 216
to rotate 360 degrees around the axis of deflectable post 204 (as shown
by arrow 234) and also tilt away from the plane perpendicular to the axis
of deflectable post 204 (as shown by arrow 236). Thus, the vertical rod
216 is allowed to rotate and/or have tilting and/or swiveling movements
about a center which corresponds with the center of the ball 214 of ball
joint 222.

[0109]FIG. 2D shows a sectional view of deflection rod 200 through the
longitudinal axis. As shown in FIG. 2D spring 206 occupies the space
between deflectable post 204 and shield 208 and is deformed by deflection
of deflectable post 204 towards shield 208 in any direction. Spring 206
applies force to the deflectable post 204 to push deflectable post 204
towards the center position. FIG. 2D also shows how deflection rod 200 is
mounted within the housing 130 of a bone anchor 102. Deflectable post is
held in a position substantially coaxial or collinear with bone anchor
102. Note that the bottom end of cavity 132 of housing 130 forms a
hemispherical pocket which receives the distal end of retainer 202.
Retainer 202 is thus trapped between spring 206 and housing 130 in a
ball-joint that allows deflectable post 204 to pivot and rotate relative
to bone anchor 102.

[0110]FIG. 2D, also illustrates the internal detail of the ball joint 222
which connects vertical rod 216 and deflectable post 204 of deflection
rod 200. The lower end of spring 206 includes spherical pocket 218 at one
end. The proximal end of the deflectable post 204 is passed through
aperture 219 in disk-shaped pocket 218 of the vertical rod 216. The
diameter of deflectable post 204 is smaller than the diameter of the
aperture 219. Once the proximal end of deflectable post 204 is passed
through the aperture 219, ball 214 is attached to deflectable post 204
using threading, fusing, gluing, press fit and/or laser welding
techniques, for example. The diameter of the aperture 219 is less than
the diameter of the ball 214 to prevent the ball 214 from passing back
through the aperture 219. Once the ball 214 is positioned within the
disk-shaped pocket 218 of the vertical rod 216, cap 220 is threaded,
fused, glued, press fit and/or laser welded, for example, into pocket 218
thereby securing ball 214 within disk shaped pocket 218.

[0111]FIG. 2E shows a partial sectional view of a fully assembled
deflection rod 200 along the axis indicated by line 2E-2E of FIG. 2C. As
shown in FIG. 2E, spring 206 occupies the space between deflectable post
204 and shield 208 and is deformed by deflection of deflectable post 204
towards shield 208 in any direction. Spring 206 resists deflection of
deflectable post 204 outwardly from a position that is collinear with the
longitudinal axis of the spring 206. Spring 206 may be described for
example as a centering spring. The spring rate of spring 206 is selected
to generate the desired deflection/load characteristics for the
deflection rod.

[0112]FIGS. 2F and 2G illustrate deflection of deflectable post 204.
Applying a force to ball-joint 222 causes deflection of deflectable post
204 relative to shield 208 (and any bone anchor to which it may be
mounted). Initially deflectable post 204 pivots about a pivot point 203
indicated by an X. In this embodiment, pivot point 203 is located at the
center of ball-shaped retainer 202. In other embodiments, however, pivot
point 203 may be positioned at a different location. For example, for
other retainer shapes disclosed in the applications incorporated by
reference herein, the retainer may pivot about a point which is at the
edge of the retainer or even external to the retainer. As shown in FIG.
2F, deflection of deflectable post 204 deforms the spring 206. The force
required to deflect deflectable post 204 depends upon the dimensions of
deflectable post 204, spring 206 and shield 208 as well as the attributes
of the material of spring 206. In particular, the spring rate of spring
206 and elements thereof (See FIG. 2B) may be adjusted to impart the
desired force-deflection characteristics to deflectable post 204.

[0113]As shown in FIG. 2G, after further deflection, deflectable post 204
comes into contact with limit surface 228 of shield 208. Limit surface
228 is oriented such that when deflectable post 204 makes contact with
limit surface 228, the contact is distributed over an area to reduce
stress on deflectable post 204 and limit surface 228. As depicted, the
limit surface 228 is configured such that as the deflectable post 204
deflects into contact with the limit surface 228, the limit surface 228
is aligned/flat relative to the deflectable post 204 in order to present
a larger surface to absorb any load an also to reduce stress or damage on
the deflectable. Additional deflection may cause elastic deformation of
deflectable post 204. Because deflectable post 204 is relatively stiff,
the force required to deflect deflectable post 204 increases
significantly after contact of deflectable post 204 with shield 208. For
example, the stiffness may double upon contact of the deflectable post
204 with the limit surface 228. In a preferred embodiment, the proximal
end of deflectable post 204 may deflect from 0.5 mm to 2 mm before making
contact with limit surface 228. More preferably, deflectable post 204 may
deflect approximately 1 mm before making contact with limit surface 228.

[0114]Thus, as load or force is first applied to the deflection rod by the
spine, the deflection of the deflection rod responds about linearly to
the increase in the load during the phase when deflection of deflectable
post 204 causes compression of spring 206 as shown in FIG. 2F. After
about 1 mm of deflection, when deflectable post 204 contacts limit
surface 228 (as shown in FIG. 2G) the deflection rod becomes stiffer.
Thereafter, a greater amount of load or force needs to be placed on the
deflection rod in order to obtain the same incremental amount of
deflection that was realized prior to this point because further
deflection requires bending of deflectable post 204. Accordingly, the
deflection rod provides a range of motion where the load supported
increases about linearly as the deflection increases and then with
increased deflection the load supported increases more rapidly in order
to provide stabilization. Put another way, the deflection rod becomes
stiffer or less compliant as the deflection/load increases.

Alternative Deflection Rods/Loading Rods

[0115]FIGS. 3A-3D illustrate an alternative deflection rod 300. FIG. 3A
shows an exploded view of alternative deflection rod 300. Deflection rod
300 includes ball-shaped retainer 302, deflectable post 304, spring 306,
shield 308 and collar 310. In this embodiment, shield 308 and collar 310
are formed in one piece; however, they may be separate components. A
mount 314 is present at the proximal end of deflectable post 304 suitable
for connecting to a vertical rod. A ball may be used in place of mount
314 as previously described. In this embodiment, mount 314 is formed in
one piece with deflectable post 304 and spherical retainer 302. In
alternative embodiments, deflectable post 304 may be formed separately
from and securely attached to one or more of mount 314 and retainer 302
by laser welding, soldering or other bonding technology. Alternatively,
deflectable post 304 may be formed separately and mechanically engage one
or more of mount 314 and retainer 302 using, for example, threads. For
example, a lock ring, toothed locking washer, cotter pin or other
mechanical device can be used to secure deflectable post 304 to one or
more of mount 314 and retainer 302.

[0116]Spring 306 is made of an elastic material which permits movement of
deflectable post 304 relative to shield 308. The spring 306 effectively
controls and limits the deflection of the deflectable post 304. Spring
306 is preferably made of a polymer or a metal. For example, spring 306
may be made of Bionate®, PEEK, Nitinol, steel or titanium. The
properties of the material and dimensions of spring 306 are selected to
achieve the desired spring rate of spring 306 and impart the desired
force-deflection characteristics to deflectable post 304. In a preferred
embodiment, spring 306 and may be elastically deformed over a range of
0.5-2 mm by deflection of the deflectable post. Spring 306 fits inside
shield 308 surrounding deflectable post 304. Spring 306 may be of the
same design as spring 206 of FIG. 2B.

[0117]In this embodiment, deflection rod 300 is configured to be assembled
with a bone anchor 320 prior to implantation of the bone anchor into a
vertebra. Bone anchor 320 comprises a threaded bone screw 322 connected
to a housing 330. The threads of bone screw 322 are designed to secure
bone anchor 320 to a vertebra and may vary in configuration to be adapted
to engage particular regions of a vertebra having greater or lesser bone
density. Alternative bone anchor configurations are illustrated in FIGS.
8A-8E.

[0118]Housing 330 has a cavity 332 oriented along the axis of bone anchor
320 at the proximal end and configured to receive deflection rod 300.
Housing 330 also has an outer surface 334 adapted for mounting a
component, e.g. an offset connector. Housing 330 may in some embodiments
be cylindrical as previously described. As shown in FIG. 3A, outer
surface 334 of housing 330 may be provided with flutes 336 or other tool
engagement features. Flutes 336 may be engaged by a driver that mates
with flutes 336 for implanting and/or removing bone anchor 320.

[0119]Referring now to FIG. 3B, which shows a perspective view of a
deflection rod 300 assembled with a bone anchor 320. When assembled,
deflectable post 304 is positioned within spring 306 of FIG. 3A; spring
306 is positioned within shield 308 of FIG. 3A. Deflectable post 304,
spring 306 and shield 308 are then placed in the cavity 332 of bone
anchor 320. Threaded collar 310 is then secured in the threaded proximal
end of cavity 332. Threaded collar 310 has two sockets 311 for receiving
the pins of a pin wrench to allow threaded collar 310 to be tightened to
threads 338 of housing 330. Threaded collar 310 is laser welded to
housing 330 after installation to further secure the components. Threaded
collar 310 secures deflectable post 304, spring 306 and shield 308 within
cavity 332 of bone anchor 320. As shown again in FIG. 3B, outer surface
334 of housing 330 may be provided with flutes 336 or other tool
engagement features. Flutes 336 may be engaged by a driver that mates
with flutes 336 for implanting and/or removing bone anchor 320.

[0120]FIG. 3c illustrates the head of an open wrench 380 for driving bone
anchor 320 into position. As shown in FIG. 3D, deflection rod 300 is
already assembled with bone anchor 320 and thus a driver may not be
inserted in the cavity of bone anchor 320. Open wrench 380 has a head 382
designed to engage the exterior surface 334 of housing 330. Exterior
surface 334 is provided with features such as flutes 336 to facilitate
engagement of housing 330 by open wrench 380. With such a tool, the
housing 330 can be engaged and rotated about the longitudinal axis of the
bone anchor 320 in order to drive the bone anchor into the bone. Open
wrench 380 may be provided with a torque limiting or torque measuring
component to facilitate installation of bone anchor 320. In alternative
embodiments, a socket, wrench, pin wrench or the like may be used to
engage housing 330 in place of an open wrench. In an alternative
embodiment, a channel may be made through the long axis of deflectable
post 304 communicating (when aligned) with a polygonal channel
penetrating into the base of bone anchor 320; an Allen wrench (or similar
driver) then may be passed through the deflectable post 304 to engage the
bone anchor 320 and drive the bone anchor 320 into the bone.

[0121]FIG. 3D shows a sectional view of deflection rod 300 as assembled
with bone anchor 320 along the line 3D-3D of FIG. 3B. Ball-shaped
retainer 302 is received in a hemispherical pocket 333 in the bottom of
cavity 332. The bottom edge of spring 306 secures ball-shaped retainer
302 within hemispherical pocket 333 forming a ball-joint which allows
post 304 to pivot and rotate around the center of ball-shaped retainer
302. Spring 306 is held in place by shield 308 which is secured to
housing 330 by threaded collar 310.

[0122]When assembled, deflectable post 304 may pivot about the center of
ball-shaped retainer 302. As shown in FIG. 3D spring 306 occupies the
space between deflectable post 304 and shield 308 and is deformed by
deflection of deflectable post 304 towards shield 308 in any direction.
Spring 306 applies force to deflectable post 304 to push deflectable post
304 towards the center position. The force is applied by spring 306 to
deflectable post 304 is dependent upon the spring rate of spring 306 and
the amount of deflection. Spring 306 is designed and/or selected to
impart the desired force-deflection characteristics to deflectable post
304.

[0123]After deflectable post 304 has deflected a certain amount,
deflectable post 304 contacts the limit surface 328 of collar 310.
Thereafter, further deflection of mount 314 requires bending of
deflectable post 304. In this region then, spring 306 and deflectable
post 304 both contribute to the desired force-deflection characteristics
of deflection rod 300. Because deflectable post 304 is relatively stiff,
the deflection rod 300 becomes substantially stiffer after contact
between deflectable post 304 and limit surface 328. Put another way, the
amount of deflection of mount 314 per additional unit of load decreases
after contact between deflectable post 304 and limit surface 328. In a
preferred embodiment, the stiffness of deflection rod 300 is increased by
two times or more upon contact between deflectable post 304 and limit
surface 328. Accordingly, the deflection rod provides a range of motion
where the load supported increases about linearly as the deflection
increases and then with increased deflection the load supported increases
more rapidly in order to provide stabilization.

[0124]FIGS. 4A-4D illustrate an alternative deflection rod 400. FIG. 4A
shows an exploded view of alternative deflection rod 400. Deflection rod
400 includes ball-shaped retainer 402, deflectable post 404, spring 406,
shield 408 and collar 410. In this embodiment, shield 408 and collar 410
are formed as separate components. A mount 414 is present at the proximal
end of deflectable post 404 suitable for connecting to a vertical rod. A
ball may be used in place of mount 414 as previously described. In this
embodiment, mount 414 is formed in one piece with deflectable post 404
and retainer 402. In alternative embodiments, deflectable post 404 may be
formed separately from and securely attached to one or more of mount 414
and retainer 402 by laser welding, soldering or other bonding technology.
Alternatively, deflectable post 404 may be formed separately and
mechanically engage one or more of mount 414 and retainer 402 using, for
example, threads. For example, a lock ring, toothed locking washer,
cotter pin or other mechanical device can be used to secure deflectable
post 404 to one or more of mount 414 and retainer 402.

[0125]Spring 406 is made of an elastic material which permits movement of
deflectable post 404 relative to shield 408. The spring 406 effectively
controls and limits the deflection of the deflectable post 404. Spring
406 is preferably made of a polymer or a metal for example, PEEK,
Bionate®, titanium, steel or Nitinol. The spring rate of spring 406
is selected to achieve the desired force-deflection characteristics for
deflectable post 404. The design, dimensions and material of spring 406
are selected to achieve the desired spring rate. In one preferred
embodiment, spring 406 is made of PEEK and may be elastically deformed to
allow from about 0.5 mm to 2 mm of travel in either direction by mount
414. Spring 406 fits inside shield 408 surrounding deflectable post 404.
Spring 406 could be replaced with a spring similar to spring 206 of FIG.
2B.

[0126]In this embodiment, deflection rod 400 is configured to be assembled
with a bone anchor 420 prior to implantation of the bone anchor. Bone
anchor 420 comprises a threaded bone screw 422 connected to a housing
430. The threads of bone screw 422 are designed to secure bone anchor 420
to a vertebra and may vary in configuration so as to be adapted to engage
particular regions of a vertebra having greater or lesser bone density.

[0127]Housing 430 of bone anchor 420 has a cavity 432 oriented along the
axis of bone anchor 420 at the proximal end and configured to receive
deflection rod 400. Housing 430 also has an outer surface 434 adapted for
mounting a component, e.g. an offset connector. Housing 430 may, in some
embodiments, be cylindrical as previously described. As shown in FIG. 4A,
outer surface 434 of housing 430 may be provided with flutes 436 or other
tool engagement features. Flutes 436 may be engaged by a driver that
mates with flutes 436 for implanting and/or removing bone anchor 420.

[0128]FIG. 4B shows an enlarged view of spring 406. As shown in FIG. 4B,
spring 406 comprises a plurality of spring elements 460. Each spring
element 460 is in the form of a leaf spring. Each spring element 460 has
a first end 462 and a second end 463 shaped to engage the shield and
maintain the orientation of the spring elements 460. Between the first
end 462 and second end 463, the spring elements curve in towards a raised
middle section 464 which is designed to engage the deflectable post 404
(see FIG. 4A). When the plurality of spring elements 460 is assembled,
the middle sections 464 define an aperture 465 sized to receive the
deflectable post 404. When assembled with deflectable post 404, movement
of flexible post 404 pushes on middle section 464 of one or more spring
element 460 causing the one or more spring elements 460 to flatten out.
The spring elements resist this deformation and apply a restoring force
to the deflection rod 404 to cause it to return to the center position.
The force applied to deflectable post 404 is dependent upon the spring
rate of spring 406 and the amount of deflection of deflectable post 404.

[0129]Spring elements 460 may be individual elements as shown, or they may
be joined together, for example at the first ends 462 and/or second ends
463. If joined together, spring elements 460 may all be connected, or may
be connected in two parts such that the two parts may be assembled from
either side of deflectable post 404 during assembly with shield 408.
Spring elements 460 may, in some embodiments, be formed in one piece, for
example, machined or molded from a single block of material. In other
embodiments, spring elements 460 may be formed as separate pieces and
then attached to one another.

[0130]The spring rate of each spring element 460 may be controlled during
design by choice of the design, dimensions and material of the spring
element 460. For example, making the material of the spring elements 460
thicker or reducing the length of the spring element 460 can increase the
spring rate of the spring element. Also, the material of the spring
element 460 may be selected to achieve the desired force-deflection
characteristics. The spring elements 460 may be identical thereby
resulting in a force-deflection curve that is substantially uniform in
all directions (isotropic). In other embodiments, the spring elements may
have different spring rates thereby allowing the force-deflection curve
of the deflection rod to be anisotropic--i.e. the deflection of
delectable post 404 has different force-deflection characteristics in
different directions.

[0131]Referring now to FIG. 4c, which shows a perspective view of a
deflection rod 400 assembled with a bone anchor 420. When assembled,
deflectable post 404 is positioned within spring 406 of FIG. 4A; spring
406 is positioned within shield 408 of FIG. 4A. Deflectable post 404,
spring 406 and shield 408 are then placed in the cavity 432 of bone
anchor 420. Threaded collar 410 is then secured in the threaded proximal
end of cavity 432. Threaded collar 410 has two sockets 411 for receiving
the pins of a pin wrench to allow threaded collar 410 to be tightened to
threads 438 of housing 430. Threaded collar 410 is laser welded to
housing 430 after installation to further secure the components. Threaded
collar 410 secures deflectable post 404, spring 406 and shield 408 within
cavity 432 of bone anchor 420.

[0132]FIG. 4D shows a sectional view of deflection rod 400 as assembled
with bone anchor 420 along the line 4D-4D of FIG. 4c. Ball-shaped
retainer 402 is received in a hemispherical pocket 433 in the bottom of
cavity 432. In this embodiment, the bottom edge of sleeve 408 secures
ball-shaped retainer 402 within hemispherical pocket 433 forming a
ball-joint which allows post 404 to pivot and rotate around the center of
ball-shaped retainer 402. Shield 408 is held in position by collar 410
thereby securing retainer 402. Spring 406 is also secured by collar 410
between collar 410 and the bottom end of shield 408.

[0133]When assembled, deflectable post 404 may pivot about the center of
ball-shaped retainer 402. Deflectable post 404 may also rotate about the
long axis of the post. As shown in FIG. 4D, spring 406 occupies the space
between deflectable post 404 and shield 408 and is deformed by deflection
of deflectable post 404 towards shield 408 in any direction. Spring 406
applies force to the deflectable post 404 to push deflectable post 404
towards the center position. The force applied by spring 406 to
deflectable post 404 depends upon the spring rate of spring 406 and the
amount of deflection. Thus spring 406 imparts the desired
force-deflection characteristics to the deflectable post 404.

[0134]After deflectable post has deflected a certain amount, deflectable
post 404 contacts the limit surface 428 of collar 410. Thereafter,
further deflection of mount 414 requires bending of deflectable post 404.
In this region the, both spring 406 and deflectable post 404 contribute
to the desired force-deflection characteristics of deflection rod 400.
Because deflectable post 404 is relatively stiff, the deflection rod
becomes substantially stiffer after contact between deflectable post 404
and limit surface 428. Put another way, the amount of deflection of mount
414 per additional unit of load decreases after contact between
deflectable post 404 and limit surface 428. In a preferred embodiment,
the stiffness of deflection rod 400 is increased by two times or more
upon contact between deflectable post 404 and limit surface 428.
Accordingly, the deflection rod provides a range of motion where the load
supported increases about linearly as the deflection increases and then
with increased deflection the load supported increases more rapidly in
order to provide stabilization.

[0135]FIGS. 5A-5C show another alternative deflection rod having a
different mechanism to secure the deflectable post to the deflection rod
and/or the bone anchor and a different spring mechanism. The mechanisms
of FIGS. 5A-5B may be adapted for use in other of the deflection rods
described herein.

[0136]FIG. 5A shows an exploded view of alternative deflection rod 500.
Deflection rod 500 includes ball-shaped retainer 502, post 504, spring
506, split-ring 538, collar 510, and mount 514. In this embodiment,
ball-shaped retainer 502 is formed in one piece with post 504. In this
embodiment deflection rod 500 is assembled with a bone anchor 520, which
comprises a bone screw 522 connected to a housing 530. Housing 530 has a
cavity 532 oriented along the axis of bone anchor 520 and configured to
receive deflection rod 500. Housing 530 also has an outer surface adapted
for mounting a component, e.g. an offset connector. As shown in FIG. 5A,
the outer surface of housing 530 is provided with flutes 531. Flutes 531
may be engaged by, e.g. an offset connector and/or a driver for
implanting bone anchor 520.

[0137]In this embodiment, retainer 502 is a ball-shaped retainer. Mount
514 is suitable for connecting to a vertical rod. A second ball may be
used in place of mount 514 as previously described. In this embodiment,
mount 514 is formed in one piece with deflectable post 504. In a
preferred embodiment, mount 514, ball-shaped retainer 502 and deflectable
post 504 are formed from a single piece of titanium. In alternative
embodiments, deflectable post 504 may be formed separately from, and
securely attached to, one or more of mount 514 and retainer 502 by laser
welding, soldering or other bonding technology. Alternatively,
deflectable post 504 may be formed separately and mechanically engage one
or more of mount 514 and retainer 502 using, for example, threads, a lock
ring, toothed locking washer, cotter pin or other mechanism.

[0138]Spring 506 fits inside cavity 532 of housing 530 surrounding post
504. Spring 506 is inserted in cavity 532 of housing 530 over post 504.
Threaded collar 510 is then secured in the threaded proximal end of
cavity 532. Threaded collar 510 has two sockets 511 for receiving the
pins of a pin wrench to allow threaded collar 510 to be tightened to
housing 530. Threaded collar 510 is laser welded to housing 530 after
installation to further secure the components. Threaded collar 510
secures spring 506 within cavity 532 of bone anchor 520.

[0139]FIG. 5B shows a sectional view of a deflection rod 500 assembled
with a bone anchor 520. Ball-shaped retainer 502 may be locked in a
ball-joint pocket in a variety of ways. Some suitable methods and devices
for locking a ball in a ball-joint assembly are disclosed in U.S. Pat.
No. 4,666,330 titled "Ball Joint Assembly" to O'Connell et al. which is
incorporated herein by reference in its entirety. As shown in FIG. 5B,
ball-shaped retainer 502 fits in pocket 534 in the bottom of cavity 532
of housing 530. Pocket 534 is generally hemispherical. The entrance
aperture 535 to pocket 534 is the same diameter as ball-shaped retainer
502. However, entrance aperture 535 includes a groove 533 which receives
a split-ring 538. Split-ring 538 has a larger diameter than aperture 535
but split-ring 538 is compressed slightly during installation. After
passing through aperture 535, split-ring 538 expands outwards to occupy
groove 533. Split-ring 538, when positioned in groove 533, reduces the
effective diameter of aperture 535 and thereby prevents removal of
ball-shaped retainer 502. Other shapes of retainer and pocket may also be
used as long as they pivotally secure the post 504 to the bone anchor 520
and allow the desired range of travel for post 504. In the deflection rod
500 of FIG. 5A-5B, no shield is needed between spring 506 and housing
530. By removing the thickness of the shield, the size/strength
properties of the device may be enhanced.

[0140]When assembled, deflectable post 504 may pivot about the center of
ball-shaped retainer 502. As shown in FIG. 5B, spring 506 occupies the
space between post 504 and housing 530. Spring 506 is compressed by
deflection of post 504 towards housing 530 in any direction. Spring 506
applies force to the deflectable post 504 to push deflectable post 504
towards the center position. The force applied by spring 506 to
deflectable post 504 is dependent upon the spring rate of spring 506 and
the amount of deflection of deflectable post 504. Thus, spring 506
initially imparts the desired force-deflection characteristics to post
504.

[0141]The interior surface of cavity 532 of housing 530 and/or collar 510
is shaped to provide the limit surface 572 to limit deflection of post
504. In a preferred embodiment, the spring may be compressed about 1 mm
by deflection of the post 504 prior to contact of post 504 with limit
surface 572 of collar 510. Thereafter, further deflection of mount 514
necessitates bending of post 504 and/or bone anchor 520. Post 504 and
anchor 520 are stiffer than spring 506, thus upon contact of post 504 and
limit surface 572, further deflection requires greater force per unit of
deflection than prior to such contact. In preferred embodiments, the
stiffness of the system increases to about double the stiffness of the
spring after contact is made between post 504 and limit surface 572.

[0142]In this embodiment, spring 506 is formed from a plurality of planar
springs 560. Spring 506 may comprise one or more planar springs 560.
Planar springs 560 may be cut or stamped from a flat sheet of material.
Spring 506 is preferably made of a biocompatible elastic polymer or
metal. For example, planar springs 560 may be made from, Bionate®,
Peek, Nitinol, steel and/or titanium. The properties of the design,
dimensions and material of the spring 506 and deflectable post 504 are
selected to achieve the desired force-deflection characteristics for
deflectable post 504. In some embodiments, the number of planar springs
560 in a particular deflection rod may be selectable such that stiffer
deflection rods have a larger number of planar springs 560 and more
compliant deflection rods have a lower number of planar springs 560. In
other embodiments, the spring rate of each spring 506 may be adjusted by
design, dimension or material changes.

[0144]The spring/spring elements in the deflection rod of FIGS. 5A-5B are
designed to elastically deform in the radial direction (relative to post
504). Alternative designs of springs may be used to control deflection of
post 504 including, for example, spring washers, Belleville washers/disc
springs, CloverDome® spring washers, CloverSprings®, conical
washers, wave washers, coil springs and finger washers. Examples of
alternative springs which may be used in the deflection rod of FIGS.
5A-5B are shown in FIGS. 5D-5G.

[0145]FIG. 5D shows an enlarged view of an alternative embodiment of a
planar spring 560d. As shown in FIG. 5D, planar spring 560d comprises an
inner ring 564d connected to a plurality of oblique lever arms 566d. The
outer ends 562d of lever arms 566d are positioned to fit within cavity
532. Inner ring 564d is sized so that aperture 565d just fits over post
504. The arrangement of lever arms 566d allows inner ring 564d to deflect
laterally with respect to cavity 532 (FIG. 5B) by deforming lever arms
566d. The lever arms 566d resist the deformation. When assembled with
post 504 and housing 530, planar spring 560d imparts a return force upon
post 504 upon deflection of post 504 towards housing 530. One or more
springs 560d may be used in the deflection rod of FIGS. 5A-5B.

[0146]FIG. 5E shows an enlarged view of an alternative embodiment of a
spring 560e. As shown in FIG. 5E, spring 560e is a coil spring. The coil
spring 560e is wound to form an inner ring 564e and an outer ring 562e.
The outer ring 562e is sized to fit within cavity 532 (FIG. 5B). The
inner ring 564e is sized so that aperture 565e just fits over post 504.
Between inner ring 564e and outer ring 562e, are a plurality of helical
coils 566e. The arrangement of coils 566e allows inner ring 564e to
deflect laterally with respect to outer ring 562e by deforming coils
566e. The coils 566e resist the deformation. When assembled with post 504
and housing 530, coil spring 560e imparts a return force upon post 504
when post 504 deflects towards housing 530 (FIG. 5B). One or more springs
560e may be used in the deflection rod of FIGS. 5A-5B.

[0147]FIGS. 5F and 5G show an enlarged view of an alternative embodiment
of a spring 560f. FIG. 5G shows a side view of spring 560f in which it
can be seen that spring 560f is a domed spring washer. The domed spring
washer 560f has an inner aperture 564f and an outer circumference 562f.
The outer circumference 562f is sized to fit within cavity 532 (FIG. 5B).
The inner aperture 564f is sized to fit over post 504. Domed spring
washer 560f has a plurality of interior and exterior cutouts 566f. These
cutouts increase the compliance of domed spring washer 560f (but reduce
stiffness). The cutouts are designed to allow the desired degree of
lateral deformation while still providing the desired spring rate. The
pattern of cutouts shown in FIG. 5F forms a clover pattern but other
patterns may be used, for example, fingers. The design of domed spring
washer 560f allows inner aperture 564f to deflect laterally with respect
to outer circumference 562f by deforming the material of domed spring
washer 560f. The material resists the deformation. When assembled with
post 504 and housing 530, domed spring washer 560f imparts a return force
upon post 504 when post 504 deflects towards housing 530. One or more
spring washers 560f may be used in the deflection rod of FIGS. 5A-5B.

[0148]FIG. 6A is a sectional view illustrating the implantation of
deflection rod 200 of FIG. 2A in a vertebra 640. As shown in FIG. 6A, a
bone anchor 102 is oriented such that is passes through pedicle 642 into
vertebral body 644 of vertebra 640. Note that the length of bone anchor
102 is selected based upon the anatomy of the patient. Thus, shorter bone
anchors are used in smaller vertebrae and longer bone anchors are used in
larger vertebrae. As shown in FIG. 6A, bone anchor 102 has shallower
threads 650 adjacent housing 130. These shallow threads 650 engage the
harder cortical bone 646 on the surface of the vertebra 640. Bone anchor
102 has deeper threads 652 towards the distal end of bone anchor 102.
These threads 652 are better suited to engage the softer cancellous bone
648 within the vertebral body 644.

[0149]As shown in FIG. 6A deflection rod 200 is mounted within bone anchor
102 such that pivot point 203 is positioned below the surface of vertebra
640. Deflectable post 204 pivots about this pivot point 203 positioned
close to or within vertebra 640. This is advantageous in that it places
pivot point 203 of deflectable post 204 closer to the vertebral body 644
and thus closer to the natural instantaneous center of rotation of the
spine. Placing pivot point 203 closer to the vertebral body 644 promotes
natural motion and reduces non-physiological forces on the bones and
strain on the system. Placing the pivot point 203 closer to the vertebral
body 644 also helps isolate bone anchor 102 from the relative motion
between vertebra 640 and the vertical rod 216 which connects one vertebra
to another vertebra. Pivot point 203 is preferably close to the surface
of the vertebra 640 and more preferably pivot point 203 is within the
vertebrae 640. Even more preferably, the pivot point 203 is positioned
within the pedicle 642 of the vertebra 640 within the natural range of
the instantaneous center of rotation of the spine. Although, in this
embodiment, pivot point 203 is positioned at the center of retainer 202,
in other embodiments, as described in the applications incorporated by
reference herein, the effective pivot point may be located at the edge of
the retainer or even outside of the retainer.

[0150]FIG. 6B shows a lateral view of a dynamic stabilization assembly
utilizing deflection rod 300 of FIG. 3A. As shown in FIG. 6B, deflection
rod 300 is installed in bone anchor 320. Bone anchor 320 is implanted in
a vertebra 640. A polyaxial screw 670 is implanted in a second vertebra
641. A vertical rod 660 is secured at one end to mount 314 of deflection
rod 300. Mount 314 in this embodiment passes through an aperture in
vertical rod 660. A threaded nut 662 secures vertical rod 660 directly to
mount 314. The rigid connection between vertical rod 660 and deflection
rod 300 provides a relatively stiff assembly. However, where greater
range of motion/less stiffness is desired, vertical rod 660 may be
connected to deflectable post 304 by a ball-joint, for example as
previously described with respect to FIGS. 1A-1B.

[0151]Vertical rod 660 is mounted at the other end to the polyaxial head
672 of polyaxial screw 670. This screw 670 may be a standard polyaxial
screw, for example, a 5.5 mm polyaxial screw available in the
marketplace. This screw 670 may, alternatively, be a bone anchor with a
polyaxial head e.g. the polyaxial head previously described with respect
to FIG. 1c. Alternatively, screw 670 may include a deflection rod and
bone anchor as described herein and a suitable connector to mount the
deflection rod to the vertical rod 660. In a preferred embodiment,
vertical rod 660 is a titanium rod 5.5 mm in diameter as used in rigid
spinal implants. The vertical rod 660 is secured to polyaxial head 672
using a threaded fitting, set screw 674, for example. The vertical rod
660 thereby supports the vertebrae while deflection rod 300 provides for
load sharing and allows relative motion of vertebra 640 relative to
vertebra 641. Thus, the dynamic stabilization assembly provides dynamic
stabilization of the spine and load sharing. The dynamic stabilization
assembly may be expanded to two or more levels using, for example, an
offset connector mounted to the housing 330 of bone anchor 320. Thus, a
modular system is provided which provides for the creation of a
multi-level dynamic stabilization assembly.

[0152]FIGS. 7A and 7B show an alternative embodiment of deflection rod 700
which includes a mount 770 for connecting the deflection rod to a
vertical rod. As shown in FIG. 7A, mount 770 includes a circular plate
774; the face of which is parallel to the longitudinal axis of
deflectable post 704. A threaded pin 772 projects from the center of
circular plate 774. Threaded pin 772 is perpendicular to the longitudinal
axis of deflectable post 704. On the face of circular plate 774
surrounding pin 772 are a plurality of radial splines 776.

[0153]Mount 770 is designed to mate with vertical rod 780 as also shown in
FIG. 7A. Vertical rod 780 has at one end a circular plate 784; the face
of which is parallel to the longitudinal axis of vertical rod 780. An
aperture 782 passes through the center of circular plate 784 and is sized
to receive threaded pin 772. Aperture 782 is perpendicular to the
longitudinal axis of vertical rod 780. On the face of circular plate 784
surrounding aperture 782 are a plurality of radial splines 786. The
radial splines of vertical rod 780 are designed to mate with and engage
the splines 776 of mount 770.

[0154]As shown in FIG. 7B, aperture 782 of vertical rod 780 is received
over threaded pin 772 of mount 770. The angle of vertical rod 780
relative to deflectable post 704 may be adjusted as shown by arrow 792.
Adjustment of the relative angle of deflectable post 704 and vertical rod
780 combined with the ability of deflectable post 704 to rotate about its
long axis (as shown by arrow 794) relative to bone anchor 720 provides
two degrees of freedom and thus sufficient flexibility of installation to
align vertical rod 780 with a bone anchor implanted in another vertebrae.
As shown in FIG. 7B, a nut 790 engages threaded pin 772 to secure plate
774 to plate 784. Splines 776 of plate 774 are arranged facing splines
786 of plate 784. When nut 790 is tightened, splines 786 engage splines
776 to prevent rotation of vertical rod 780 about pin 772. Thus, when the
nut 790 is tightened, the angle between deflectable post 704 and vertical
rod 780 is fixed. The vertical rod mounting mechanism of FIGS. 7A and 7B
may be readily applied to any of the deflection rod systems described
herein.

Alternative Bone Anchors

[0155]FIGS. 8A though 8E illustrate some possible variations in bone
anchors of the anchoring system. The bone anchors each have a housing
compatible with the deflection rods of the deflection system and the
offset heads/connectors of the connector system. In some embodiments, the
deflection rod is installed/assembled in the bone anchor prior to
implantation of the bone anchors in the body. In alternative embodiments,
the bone anchors may be implanted in the body before installation of a
deflection rod.

[0156]Bone anchor 810 of FIG. 8A is a bone screw having a threaded region
814 which extends up over most of a housing 812. A deflection rod 804 is
installed in housing 812. The threaded region 814 may extend over a
greater or lesser amount of housing 812 depending upon such factors as
the length of the bone screw, the type of bone in which the screw is to
be implanted and the desired height to which the housing 812 will extend
above the bone surface after implantation. Bone anchor 810 may be useful
to lower the depth of the pivot point of the deflection rod 804 closer to
the natural instantaneous center of rotation of the spine. Note also that
the distal thread depth 816 may be deeper than the proximal thread depth
818. The distal threads 818 are adapted for engagement of the soft
cancellous bone while the proximal threads is adapted for engagement of
the harder cortical bone at the surface of the vertebra.

[0157]Bone anchor 820 of FIG. 8B is a bone screw in which the screw-only
section 824 is shorter in length than in bone screw 810 of FIG. 8A. A
deflection rod 804 is installed in housing 822. Different lengths of
screw-only section may be useful in different patients or different
vertebrae as the size of the bone in which the anchor needs be implanted
may vary considerably. For example, short bone screws are desirable where
the dynamic stabilization system is to be implanted in smaller vertebrae.
The physician may determine the length of bone screw appropriate for a
particular patient by taking measurements during the procedure by
determining measurements from non-invasive scanning, for example, X-ray
NMR, and CT scanning. Note however, that housing 822 is preferably the
same size and shape as the housings of the other bone anchors so as to be
compatible with the same deflection rods, components and connectors.

[0158]Bone anchor 830 of FIG. 8c is a bone screw in which the screw-only
section 834 has a smaller diameter and is shorter in length than in bone
screw 810 of FIG. 8A. A deflection rod 804 is installed in housing 832.
Different diameters of screw-only section may be useful in different
patients or different vertebrae as the size of the bone in which the
anchor needs be implanted may vary considerably. For example smaller
diameter bone screws may be desirable where the dynamic stabilization
system is to be implanted in smaller vertebrae. The physician may
determine the diameter of bone screw appropriate for a particular patient
by taking measurements during the procedure of by determining
measurements from non-invasive scanning, for example, X-ray NMR, and CT
scanning. Note however, that housing 832 is preferably the same size and
shape as the housings of the other bone anchors so as to be compatible
with the same deflection rods, components and connectors.

[0159]Bone anchor 840 of FIG. 8D is a bone screw in which the housing 842
has a rim 844 extending away from housing 842 where it transitions to the
threaded region 846. A deflection rod 804 is installed in housing 842.
Rim 844 may serve to retain an offset head mounted to housing 842 in a
way that it can rotate freely around housing 842 during installation. Rim
844 may also serve to widen the contact area between the bone anchor 840
where it meets the bone of the vertebra. This can act as a stop
preventing over-insertion. This can also provide a wide base for
stabilizing the housing against lateral motion and torque. Note that
housing 842 is preferably the same size and shape as the housings of the
other bone anchors so as to be compatible with the same deflection rods
and connectors.

[0160]Bone anchor 850 of FIG. 8E illustrates a bone hook device 851 having
a housing 852. A deflection rod 804 is installed in housing 852. Bone
hook device 851 comprises a bar 854 to which housing 852 is rigidly
connected. At either end of bar 854 is a bone hook 856 having a set screw
859 for securing the bone hook 856 to the bar 854. Each bone hook 856 has
a plurality of sharp points 858 for engaging and securing the bone hook
856 to a vertebra. During use, the bone hooks 856 are urged towards each
other until the sharp points engage and/or penetrate the surface of a
bone. Set screws 859 are tightened to secure bone hooks 856 in position
relative to bar 854 and thus secure housing 852 relative to the bone.
Different arrangements of bone hooks and bars may be made suitable for
attachment of the housing 852 to different types, sizes, shapes and
locations of vertebra. Note that housing 852 is preferably the same size
and shape as the housings of the other bone anchors so as to be
compatible with the same deflection rods, components and connectors.

Deflection Rod/Loading Rod Materials

[0161]Movement of the deflectable post relative to the bone anchor
provides load sharing and dynamic stabilization properties to the dynamic
stabilization assembly. As described above, deflection of the deflectable
post deforms the material of the spring. The spring applies a restoring
force upon the deflectable post; the force being dependent upon the
spring rate of the spring and the amount of deflection of the deflectable
post. The design, dimensions and the material of the spring may be
selected to achieve the desired spring rate. The characteristics of the
spring in combination with the dimensions of the other components of the
deflection rod interact to generate the force-deflection curve of the
deflection rod.

[0162]The design, dimensions and materials may be selected to achieve the
desired force-deflection characteristics. By changing the dimensions of
the deflectable post, spring and spring elements the deflection
characteristics of the deflection rod can be changed. The stiffness of
components of the deflection rod can be, for example, increased by
increasing the diameter of the deflectable post. Additionally, decreasing
the diameter of the deflectable post will decrease the stiffness of the
deflection rod. Alternatively and/or additionally, changing the materials
which comprise the components of the deflection rod can also affect the
stiffness and range of motion of the deflection rod. For example, making
the spring out of stiffer and/or harder material increases the load
necessary to cause a given deflection of the deflection rod.

[0163]The deflectable post, bone anchor and vertical rods are preferably
made of biocompatible implantable metals. The deflectable post can, for
example, be made of, titanium, a shape memory metal for example Nitinol
(NiTi) or stainless steel. In preferred embodiments, the deflectable post
is made of titanium or cobalt chrome. In preferred embodiments, the bone
anchor and vertical rods are also made of titanium; however, other
materials, for example, stainless steel may be used instead of or in
addition to the titanium components. The ball of the vertical rod may
also be made of cobalt chrome for its improved wear characteristics.

[0164]The spring can be formed by extrusion, injection, compression
molding and/or machining techniques, as would be appreciated by those
skilled in the art. In some embodiments, the spring is formed separately.
For example, a spring may be cut or machined from a biocompatible polymer
and then assembled with the deflectable post and spring such as by being
press fit into the shield. Alternatively or additionally, a fastener or
biocompatible adhesive may be used to secure the spring to the shield
and/or post.

[0165]The material of the spring is preferably a biocompatible and
implantable polymer or metal having the desired deformation
characteristics--elasticity and modulus. The material of the spring
should also be able to maintain the desired deformation characteristics.
Thus the material of the spring is preferably durable, resistant to
oxidation and dimensionally stable under the conditions found in the
human body. The spring may, for example be made from a PEEK or a
polycarbonate urethane (PCU) such as Bionate® or a surgical steel or
titanium or Nitinol. If the spring is comprised of Bionate®, a
polycarbonate urethane or other hydrophilic polymer, the spring can also
act as a fluid-lubricated bearing for rotation of the deflectable post
relative to the longitudinal axis of the deflectable post.

[0166]Other polymers or thermoplastics may be used to make the spring
including, but not limited to, polyether-etherketone (PEEK),
polyphenylsolfone (Radel®), or polyetherimide resin (Ultem®).
Other polymers that may be suitable for use in some embodiments, for
example, other grades of PEEK, for example 30% glass-filled or 30% carbon
filled, provided such materials are cleared for use in implantable
devices by the FDA, or other regulatory body. Glass-filled PEEK is known
to be ideal for improved strength, stiffness, or stability while carbon
filled PEEK is known to enhance the compressive strength and stiffness of
PEEK and lower its expansion rate.

[0167]Still other suitable biocompatible thermoplastic or thermoplastic
polycondensate materials may be suitable, including materials that have
good memory, are flexible, and/or deflectable have very low moisture
absorption, and good wear and/or abrasion resistance, can be used without
departing from the scope of the invention. These include
polyetherketoneketone (PEKK), polyetherketone (PEK),
polyetherketoneetherketoneketone (PEKEKK), and polyetheretherketoneketone
(PEEKK) and generally, a polyaryletheretherketone. Further, other
polyketones can be used as well as other thermoplastics.

[0168]Still other polymers that can be used in the spring are disclosed in
the following documents, all of which are incorporated herein by
reference. These documents include: PCT Publication WO 02/02158 A1, dated
Jan. 10, 2002 and entitled Bio-Compatible Polymeric Materials; PCT
Publication WO 02/00275 A1, dated Jan. 3, 2002 and entitled
Bio-Compatible Polymeric Materials; and PCT Publication WO 02/00270 A1,
dated Jan. 3, 2002 and entitled Bio-Compatible Polymeric Materials.

[0169]The design, dimensions and materials of the spring are selected in
combination with the design of the deflection rod to create a deflection
rod having stiffness/deflection characteristics suitable for the needs of
a patient. By selecting appropriate spring and spring rate the deflection
characteristics of the deflection rod can be configured to approach the
natural dynamic motion of the spine of a particular patient, while giving
dynamic support to the spine in that region. It is contemplated, for
example, that the deflection rod can be made in stiffness that can
replicate a 70% range of motion and flexibility of the natural intact
spine, a 50% range of motion and flexibility of the natural intact spine
and a 30% range of motion and flexibility of the natural intact spine.
Note also, as described above, in certain embodiments, a limit surface
cause the stiffness of the deflection rod to increase after contact
between the deflectable post and the limit surface.

[0170]The foregoing description of preferred embodiments of the present
invention has been provided for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise forms disclosed. Many embodiments were chosen
and described in order to best explain the principles of the invention
and its practical application, thereby enabling others skilled in the art
to understand the invention for various embodiments and with various
modifications that are suited to the particular use contemplated. It is
intended that the scope of the invention be defined by the claims and
their equivalents.